2002 Ktm

[John Curea]

The biggest problem I see with the PDS spring is the fact of the obvious coil binding of the lighter portion of the spring. I just cant figure out how you are going to have any kind of smooth spring action with coil binding taking place at the crossover point......:think

 

[MikeS]

:confused: I don't want to spoil this tread with a basic question but how does this coil bind screw things up...I am familar with Works Per. way of the spacer ring making a solid link to the main spring at cross over. Wouldn't the coils becoming solid do the same thing ? I know coil bind can fatigue the spring.

 

[marcusgunby]

Is that why the new PDS range is 265mm long -to prevent coil bind?

 

[cp380sx]

Coil bind is when the entire spring is at it's limit. The lighter part of a progressive spring is supposed to bottom before the heavier windings do their thing. There is nothing wrong with having the the lighter area compressed to their limit as long as there is still travel left in the heavier windings.

Also, I believe the Race-Tech suggestion (PDS-4) pertains to 250's and up.

 

[MikeS]

Marcus

I haven't had them on a spring tester yet but the longer spring may provide a different crossover point along with allowing a better sag adjustment. I have no idea how this directly relates to the PDS valving. I have spent alot of time fooling with the Work Performance dual spring system. With 3 different short springs and 4 mains and a customizable crossover spacer you can get some neat results.

 

[Onore GT]

The reason for the longer spring has to do with reliability and manufacturing tolerances I believe. It is difficult for spring manufacturers to wind springs for production purposes that have a wide range of progression with any consistancy between springs. By specifying a longer spring, more progressive springs can be produced, the progression rate stays more consistant, and the spring is more resistant to sacking.

 

[svi]

The PDS gives velocity and position sensitive damping, what are the positional damping requirements that they feel can be achieved better within the shock than externally through a linkage.

 

[Jeremy Wilkey]

WOW.. While I've been away the forum has been smoking..


Couple of things. there are SO MANY ISSUES...

First the RT Graph.. My intial responce to this was cool, but huh? The graph is miss leading. As several of you have already mentioned the issues are greater than this..

Whats the old saying, liares may figure, and figures may lie.. The graph shows the relationship of Force to bottom but don't actually adress any damping related issues.. So comaparing force applied and how it aquates to postion without damping considerations as well is only a little peice of the picture..

By the looks of it they may be attempting to go the route of the single piston.. I spent lots of time working on a single piston and utimaltey proved myself wrong ,a spring like RT's may have helped.

The fact that the WP shock has such a linear curve is what makes it work so well. On the CR250 shock we have a rise not only in force to bottom but inertia related damping. Put more simply the rise in the curve is due to a decrease in leverage. This means the shock moves faster and becomes stiffer along with effectively increasesing spring rate..

Now the part about a increase in mid stroke restance.. I ask whats fundementaly wrong with this, espicaly if we don't have a incresase in damping? What do most well designed after market linkages acomplish?
My argument is also this, the stifer mid range actualy helps keep the rear suspension up in the stroke and prevents a rapid change in feel from say a bump that you hit accelrating out of a corner with the weight on the rear, and say the normally huge spike you would get if your driving through a g-laod.. The PDS system shines in these situations..

Bottoming? Screw the difference is spring numbers we can double the compression damping over just enough travel to prevent a spike.. And or run a stiffer bottoming bump..

Ok as for the PDS shaft speed.. Along time ago when Marcus and I got to speaking with each other I suspected it as a problem.. The slower speed lends itself to a system that must pass a large volume of fluid through a series of passages. Also the slower speed amplifes the advanatge of the dual piston. The shape of the needle would have to be changed to a longer more tappered set-up to prevent a "Sharp" spike in feel if the dampers speed increased. Also the issue of cross over would become worse..


The only thing that really comes to mind is that the shocks ablity to respond to compreesion chages at very low speeds is signifacly impacted by the rebound adjuster.. Tweaking needle shape, or oil viscosity may change the overlaps and reduce this problem "if" it even exists..


As for nose pressure.. The shock shaft is compressing the bladder or piston in a shock. The force required to compress this piston is a fucntion of the areas between them.. So think about it like this..

Following very simple hydraulic principles (over simplified for the example), the rod, even though it has say 180lbs of nitrogen behind it, has an area of 1/10th the bladder, so it could have about 18 lbs. of extension force.


The WP shock is diffrent in that the Rod is 18mm and the area of the piston is 46mm.. This results in more nose pressure that can be in some ways compensated for by spring chnages..Although this is not such good idea on long travel shocks like bikes but this is less of a problem techincally than say on a CR125.. Also beacuse the stroke is shorter than say that of a CR shock the subsequent pressure rise is also smaller from full exstension to full compression which means that it is more of a "true" constant which again can be considered a increase or decrease of spring..

Mace made me a sweet spread sheet that caluclates out spring force based on rate, compreesable length and prelaod.. Very cool deal actualy.. This makes it very easy to model difrent options..


Well I'm sure more will come !!


Jer